Over the last decade, an intimate relationship between cells of the skeletal and immune systems has come under appreciation. This new field has been termed osteoimmunology. A disturbance in the relationship between immune cells, bone forming osteoblasts (OB) and bone resorbing osteoclasts (OC) results in diseases such as osteoporosis, bone tumors and arthritis. Despite the common occurrence of these conditions in the elderly, osteoimmunology in aged animals has been understudied. As the worldwide population ages, these problems will become more common and the clinical and social implications created by this dearth of knowledge will grow in significance. This grant seeks to establish a better understanding of how aging affects osteoimmunology by leveraging recently developed models of OC biology and an exciting observation that germ-free (GF) mice are protected from age related osteoporosis. OCs differentiate from myeloid precursors under the influence of the cytokine RANKL, derived from osteocytes, activated T-cells or cytokine stimulated stromal cells. Overactive OCs underlie the skeletal abnormalities observed in osteoporosis, arthritis and bone malignancy. For years, the identity of the osteoclast precursor (OCP) was poorly defined. Recently, we showed that OCPs comprise a minor population of CD11blowLy6chigh myeloid cells. Interestingly, these OCPs exhibit a surprising second function: they are capable of potently suppressing T cell responses in vitro and in the setting of inflammatory arthritis in vivo. The first two aims of this grant evaluate the hypothesis that age related changes in CD11blowLy6chigh OCPs underpin two universal phenomena of aging: bone loss and reduced cellular immunity. The final aim comes on the heels of a recent explosion in our collective understanding of how the microbiome influences the immune system and metabolism to cause disease. However, relatively little is known about how the microbiota influences bone biology. In collaboration with Dr. Balfour Sartor at the National Gnotobiotic Rodent Resource Center at UNC we have generated preliminary data suggesting that the microbiome is a determinant of age-related osteoporosis. Aim 3 of this grant will determine how manipulation of the microbiome in aged mice changes bone mass and whether the microbiome from aged mice is capable of inducing osteoporosis in young animals. Strengths of this proposal include the significant problem it addresses (age related declines in bone quality and immunity), the diverse team of investigators with expertise in OC biology, immunology and the microbiome, the application of state of the art mouse models of osteoimmunology and innovative hypotheses regarding the effect of senescence on OCP biology and microbiome associated bone loss. These studies will further our understanding of how aging affects the intersection of bone biology and immunity, and may lead to new treatment strategies for osteoporosis, arthritis, cancer and infection.